1. Field of the Invention
The present invention relates to a method for forming post spacers in a liquid crystal display, and more particularly to a method for forming post spacers in a liquid crystal display, which enables cell gap to be uniform by means of a half-tone mask.
2. Description of the Prior Art
A liquid crystal display is a device in which a liquid crystal layer consisting of liquid crystal molecules is sandwiched between two substrates each having a transparent electrode formed thereon, and the arrangement of the liquid crystal molecules is changed by an electric field occurring between the transparent electrodes so that the transmission of light is controlled, thereby displaying desired images.
In this liquid crystal display, characteristics including response time, contrast ratio, viewing angle and luminance uniformity are closely connected with the liquid crystal layer thickness (i.e., cell gap), maintaining a uniform cell gap is very critical to improve image quality. In particular, as the liquid crystal display is gradually increased with respect to area and quality, the maintenance of the uniform cell gap is of more importance.
For this reason, in a process for fabricating the current liquid crystal display, for example in an assembling process between two substrates, spacers for maintaining the cell gap is mostly scattered on either of the substrates. In this case, a manner of scattering the spacers includes a dry manner in which the spacers are charged and uniformly scattered by repulsive power between the spacers of the same polarity, and a wet manner in which the spacers are mixed in a solvent such as IPA and scattered.
In a thin film transistor-liquid crystal display (TFT-LCD), the dry manner having a characteristic of excellent scattering uniformity is generally employed.
However, in view of tendency toward larger area and higher quality of the LCD device, the manner of maintaining the cell gap by scattering the conventional spacers has many processing problems.
Namely, in the spacer-scattering manner, uniform scattering of the spacers is difficult, the aggregative phenomenon of the spacers occurs, and the light leakage phenomenon occurs surrounding the spacers, so that it is difficult to uniformly maintain the cell gap. This results in the deterioration of image quality.
Accordingly, in another attempt to maintain the cell gap, there was proposed a method in which post-type spacers are formed on a color filter substrate or a TFT substrate.
FIG. 1 is a schematic view showing a general color filter, FIG. 2 is an enlarged view of the region A of FIG. 1, and FIG. 3 is an enlarged view of the region B of FIG. 2.
As shown in FIGS. 1 to 3, an active region 10 and a dummy region 20 are defined in a color filter substrate 1, after which a black matrix region 11 for blocking light is formed on the substrate 1 at the outside of the active region 10. Color pixels 12a, 12b and 12c are selectively formed on the active region 10, and at the same time, dummy color pixels 12a′, 12b′ and 12c′ are formed in order to stably pattern the color pixels 12a, 12b and 12c and to ensure a process margin, such as rubbing in a cell process. Also, post spacers 13a are uniformly formed in the color filter substrate 1 at a uniform density. At this time, the black matrix is 2 to 5 mm in width, and the dummy color pixels 12a′, 12b′ and 12c′ are formed at intervals of about 0.5 to 3 pixels.
Meanwhile, in order to make a post density uniform on the color filter substrate 1 having the post spacers 12 formed therein, the post spacers 13a are placed on the active region 10 and also on the color pixels 12a, 12b and 12c of the black matrix 11. As a result, a height difference between the post spacers 13a is caused, as shown in FIG. 4.
In FIG. 4, h1 indicates a step height caused by the black matrix 11 of a different width when forming the red pixel 12c and a protective film 14, and is about 0.2 to 0.5 μm where the resin black matrix 11 is applied. h2 indicates a step height caused by a patterned lower region of a different width when pattering a material for forming the post spacer 13a, and is about 0.1 to 0.4 μm. h3 indicates an entire step height between the two regions occurring at the end of the process and is about 1 μm or below.
Namely, the prior method for forming the post spacers in the liquid crystal display as described above has the following problems.
Due to the height difference between the post spacer formed on the active region and the post spacer formed on the outer black matrix, a cell gap difference between the active region and the outer black matrix occur, as shown in FIG. 5, when assembling the color filter substrate 100 and the array substrate 30 together with a sealant. In FIG. 5, d1 is cell gap in the active region, and d2 is a cell gap in an RGB dummy pattern portion of the outer black matrix. Thus, the luminance non-uniformity of the liquid crystal display is caused.